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Batteries

Explanation of common battery technologies seen in the computer industry.

Battery technologies are often overlooked by most of the population when computer equipment is being purchased. This includes network administrators. Many networks operate with a large number of laptops and/or PDAs like the Blackberry or a Palm Handheld. So why don't more people spend the time to research and compare the battery technologies? I believe that this is because most people don't understand batteries. This is understandable because the topic is complex and people can become overwhelmed when they start seeing all of the long and sometimes incomprehensible chemical names and symbols.

In this article I will review and explain a few technologies behind some of the batteries you might encounter. I will attempt to do this using as few technical chemical terms as possible.

Lithium ion batteries

Let's start by examining a case which has recently been in the news. In 2006 laptop batteries in Apple and Dell computers were recalled. You many remember that there was a little problem with some batteries catching fire, like the one shown in Figure 1. Figure 1 shows a Dell laptop which caught fire because of a defective Sony.

In this case the batteries were a type referred to as lithium ion. This means that inside the battery the electricity is transferred via lithium ions. These lithium ions are contained in a flammable liquid between two terminals. One terminal is negatively charged and one terminal is positively charged. When an device, like a laptop, connects these terminals, electricity flows because of the chemical reactions with the lithium ions.

So how did the batteries catch fire? Well, it all comes down to the manufacturing process for lithium ion batteries. The manufacturing process causes small pieces of metal to be within the flammable liquid that contains the lithium ions. No manufacturer has been able to prevent this. There are things that can be done. For example, engineers can refine processes to limit the number and size of the metal pieces. When the battery is in use, and begins to operate at a higher temperature, these metal pieces will move around. Depending on the precise operating conditions, as well as the size and number of metal pieces, there is an increased risk that the these metal pieces will puncture the separator. This separator, as it's name implies, separates the positive and negative terminals. When this separator is punctured, a short circuit will occur and your battery will catch fire.

Lithium ion advantages

Despite the risk of fire (which is really quite a small risk), lithium ion batteries are extremely popular in consumer electronics, especially laptops. This is because there are many advantages to using a lithium ion battery. One advantage is that lithium has a very small atomic mass thereby reducing the weight of the battery. Combined with this fact is that lithium is extremely reactive so it can give off a tremendous amount of energy in its chemical reactions. These two factors result in a very high energy density. Manufacturers of laptops and PDAs prefer to build their products with batteries having high energy density because it results in smaller, lighter, products.

When not in use, lithium ion batteries hold their charge very well, losing only about 5 percent of their charge in a month. There is also no memory effect associated with this technology. This means that you do not have to drain the battery before recharging it. Not only do you not have to drain your lithium ion battery, but you can also expect hundreds of charge/discharge cycles before losing performance.

Lithium ion disadvantages

There are some disadvantages to lithium ion batteries, apart from small risk of fire that is. One major disadvantage is that lithium ion batteries have a shelf life of 2-3 years, whether you use them or not. So if the battery in your laptop has been sitting in a warehouse for a year before making it to your laptop, you will notice a significantly shortened life span. When purchasing these batteries you need to make sure that not a significant amount of time has passed since its date of manufacture.

Lithium ion batteries also require sufficiently complex on-board circuitry to manage the battery performance. This circuitry adds additional costs to the battery which other technologies don't have. Why do they need this circuitry? There are a number of reasons, one of which is to ensure that the battery is not completely drained of its charge. If a lithium ion battery is completely discharged, it will be useless. When the charge becomes low, the on-board circuitry will stop the battery's operation to prevent complete discharge.

Lithium polymer cell batteries

An alternative to lithium ion batteries is the lithium polymer cell. This technology is extremely similar to the lithium ion technology. The major difference is that instead of the battery containing a liquid, it contains a solid. It's not really important to know how this is done, just that it results in some pretty significant advantages.

Lithium polymer advantages

One advantage to lithium polymer batteries is that they are more resistant to physical trauma. There is also lower manufacturing costs associated with lithium polymer cells.

Another advantage is that lithium polymer batteries can be easily shaped to fit many different devices. This can be done because a rigid outer casing is not required. In a lithium ion battery, the rigid, metal, outer case applies pressure which holds the terminals to the separators. In a lithium polymer battery, the separator and terminal are laminated together, eliminating the need for the rigid metal case. Without this metal casing, the lithium polymer batteries are also much lighter. And since the lithium polymer cells are more resistant to physical trauma, there is no danger in losing the protection provided by the metal casing.

Figure 2 shows a battery developed at NASA which uses lithium polymer technology. I'm sure we can all imagine some unique applications where NASA would need to take advantage of the weight and shaping advantages provided by the lithium polymer technology. Personally, I think if it's good enough for NASA, it's good enough for me.

Compared with lithium ion batteries, lithium polymer batteries also allow for more charge/discharge cycles. Typically you can expect over 500 charge/discharge cycles before the capacity is reduced to 80 percent.

Because of all of these advantages, lithium polymer batteries have quickly become popular in mobile consumer electronics like PDAs, cellphones, and iPods. You can also find some laptops with this technology, including Apple's MacBook Pro.

Thin film lithium batteries

A newer variant from the lithium ion technologies is called thin film lithium. This technology is currently only of benefit to specialized applications, and will not, for some time, become common in laptops or PDAs. I'm including it here to demonstrate to you the potential in the lithium ion technology. We're in for some great products in the next few years.

Lithium iron phosphate batteries

Another alternative to the standard lithium ion battery is the lithium iron phosphate battery. This technology was developed to solve the fire problem with lithium ion batteries which I mentioned earlier. It does this because the chemical bond, because of the iron and phosphate, is much stronger and won't break down under short circuit conditions.

Of course, there are some major disadvantages to lithium iron phosphate batteries. The most undesirable of these disadvantages is the low energy density. A low energy density means that in order to get comparable performance a much larger battery is required.

So this covers some of the battery technologies you might come across. I hope I've accomplished my goal of explaining these technologies without using too many complex technical terms. If you have any questions, or would like me to write about another technology, please feel free to send me an email.

See Also

The Author — Russell Hitchcock

Russell Hitchcock is a consultant whose specialties include networked hardware, control systems, and antennae.

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